1. Field of the invention
The present invention relates to indigestible dextrin prepared by heat-treating potato starch with addition of an acid.
2. Description of the Prior Art
Heat-treated starch (pyrodextrin) is prepared by heating a starch containing several percent water in the presence or absence of an acid. The glucose forming the starch consists primarily of 1.fwdarw.4 and 1.fwdarw.6 glycosidic linkages with very few 1.fwdarw.3 and 1.fwdarw.2 glycosidic linkages as is already known.
The proportion of these glycosidic linkages are disclosed only in J. D. Geerdes et al., J. Am. Chem. Soc., Vol. 79, 4209(1957), G. M. Christensen et al., J. Am. Chem. Soc., Vol. 79, 4492(1957) and the literature mentioned below. Commercial potato starch as heated-treated with addition of hydrochloric acid comprises at least 57.3% 1.fwdarw.4 glycosidic linkage fraction (2,3,6-Tri-O-Methyl-D-glucose), 2.6% 1.fwdarw.6 glycosidic linkage fraction (2,3,4-Tri-O-Methyl-D-glucose), up to 1.2% 1.fwdarw.3 glycosidic linkage fraction (2,4,6-Tri-O-Methyl-D-glucose), and 6.3% of a fraction having both 1.fwdarw.4 and 1.fwdarw.6 linkages (2,3-Di-O-Methyl-D-glucose).
R. L. Whistler and E. F. Paschall, Starch Chemistry & Technology, Vol. 1, p. 430(1965) makes reference to analyzed values of linkage types constituting heat-treated amylopectin and heat-treated amylose obtained by separating corn starch into amylopectin and amylose fractions and individually heating the fractions with addition of an acid. The analyzed values obtained for the heat-treated fractions were prepared by gelatinizing corn starch, then separating the starch into the two fractions and heating each fractions. The crystalline structure of powder heat-treated thus differs from that of natural starch, so that the values can not be directly used for comparison. However, in view of the fact that the ratio between the two fractions of usual corn starch is about 8:2, the analyzed values, when calculated for corn starch correspond to 67% 1.fwdarw.4 glycosidic linkage fraction (2,3,6-Tri-O-Methyl-D-glucose), 2.7% 1.fwdarw.3 glycosidic linkage fraction (2,4,6-Tri-O-Methyl-D-glucose), and 7.8% a fraction having both 1.fwdarw.4 and 1.fwdarw.6 linkages (2,3-Di-O-Methyl-D-glucose).
U.S. Pat. No. 2,274,789 discloses a prior-art process for preparing heat-treated starch and an apparatus therefor. This patent discloses a continuous treatment wherein hydrochloric acid is continuously added to a starch with a water content of about 45%, the starch dried in the form of a fluidized bed, heated at 105.degree. to 260.degree. C. and cooled to obtain a product of stable quality free of the hazard of explosion.
U.S. Pat. No. 2,332,345 discloses a process wherein starch is heated at 120.degree. to 176.degree. C. with stirring in a cylindrical container equipped with a steam jacket to obtain a product which has excellent properties as to adhesion, body smoothness of paste, stability and permanence of paste fluidity and shortened time of drying.
U.S. Pat. No. 2,565,404 discloses a process for preparing a starch sugar easily by adding hydrochloric acid to a powdery or wet starch, heating the starch to about 100.degree. C. in an autoclave equipped with a stirrer while introducing a gas into the autoclave and heating the starch to about 130.degree. C. with introduction of steam.
U.S. Pat. No. 2,698,818 discloses a process for preparing a product having a high quality over a wide range by drying starch in a vacuum within an autoclave with stirring and heating, and further heating the starch to 168.degree. C. in a vacuum with introduction of hydrogen chloride gas.
U.S. Pat. No. 2,818,357 discloses a process which comprises adding hydrochloric acid to starch, then feeding the starch into a heated spiral coil, and heating the starch to 115.degree. to 160.degree. C. within a short period of time while moving the starch through the spiral coil by vibrating the coil, whereby a uniform product is continuously prepared with a high thermal efficiency.
U.S. Pat. No. 2,845,368 discloses a process wherein vaporized hydrochloric acid is added to starch while the starch is being fluidized by introduction of a gas so as to react the starch under widely varying conditions by heating the starch in the fluidized state within a short period of time.
U.S. Pat. No. 3,200,012 discloses a process comprising adding hydrochloric acid to starch in the form of fine particulate and feeding the starch into a rotatable drum internally provided with a heating tube to heat the starch and continuously obtain a product having a diminished reducing sugar content and good solution stability.
Furthermore, Tomasik, P. and Wiejak, S., Advance in Carbohydrate Chemistry, Vol. 47, 279-343(1990) generally describes the latest information as to heat-treated starches.
When analyzed, the products of the foregoing processes were found to contain up to 30% indigestible fraction. When starch was heated under altered conditions to obtain a higher indigestible content, it was possible to increase the content to about 60%, whereas the product then contained an increased amount of colored substance, had a stimulative odor, therefore required purification, and was not practically useful because of extreme difficulties encountered in refining the product. Accordingly, the conventional processes fail to afford a product which contains about 60 to 90% indigestible fraction as contemplated by the present invention.
As to the decomposition of carbohydrates with use of an extruder, U.S. Pat. No. 4,316,747 discloses a process wherein a cellulose slurry containing an acid added thereto is heated in a twin-screw extruder under an increased pressure to prepare glucose.
With improvements in living standards in Japan in recent years, eating habits have changed and become similar to those of American and European people. This trend has resulted in a lengthened average life span and a rapidly aging society with marked increases in degenerative diseases. Manifestly, people have become health-oriented. Attention has, therefore, been directed to dietary fibers and oligosaccharides enhance the function of foods and livestock feeds in that these materials are known to alleviate constipation and other desired biological regulatory functions.
Indigestible substances, such as dietary fibers and oligosaccharides, exhibit various modes of behavior in the digestive tracts producing physiological effects on the living body. First in the upper digestive tract, water-soluble dietary fibers slow the transport of food and delay the absorption of nutrients. Delayed absorption of sugar, for example, suppresses the rise in blood sugar value, consequently lowering insulin requirements. Further, excretion of bile acid is promoted, diminishing the sterol group in the body lowering the cholesterol level of the serum. Other physiological effects through the endocrine system of the body are also reported.
Another feature of these indigestible substances is they are not digested or absorbed by the digestive tract, including the small intestine and reach the large intestine. On reaching the large intestine, oligosaccharides and dietary fibers are partly acted on by enterobacteria yielding short-chain fatty acids, intestinal gases, vitamins, etc. Acidification of the intestinal environment by the short-chain fatty acids condition the intestine. It has also been reported that when absorbed, these short-chain fatty acids are metabolized to provide energy and, simultaneously, inhibit the synthesis of cholesterol. Therefore, ingestible substances are necessary in obtaining these desired physiological effects.
A "dietary fiber hypothesis" suggested by Trowell and Burkitt epidemilogically revealed that there Is a negative correlation between the intake of dietary fibers and the onset of non-infectious diseases such as cholelithiasis, ischemic heart diseases, cancer of the large intestine, etc. Thus, insufficient ingestion of dietary fibers is thought to be a cause of degenerative diseases which are said to be diseases of the Western type. The dietary fibers are defined as the "whole group of indigestible components of foods which are not digestible by human digestive enzymes" and are classified into insoluble dietary fibers and water-soluble dietary fibers according to the solubility in water. Of these, water-soluble dietary fibers have attracted attention as materials for functional foods and livestock feeds because of their great physiological function.
For example, it is said that high viscosities inhibit diffusion of sugar, resulting in delayed absorption of sugar and reduction in the rise of blood sugar value, consequently lowering insulin necessity. Further it is said that promoted excretion of bile acid into feces by water-soluble dietary fibers diminishes cholesterol in the serum, and that after reaching the large intestine, the dietary fibers are acted on by enterobacteria to produce lactic acid and acetic acid with these organic acids lowering the pH within the large intestine and preventing cancer of the large intestine.
Examples of such water-soluble dietary fibers include guar gum, glucomannan, pectin and like natural gums which have high viscosity which are difficult to ingest singly in a large amount. Further the addition of these fibers to processed foods encounters problems in preparing the food and presents difficulties with respect to texture. It has therefore long been desired to provide dietary fibers of low viscosity which have the same physiological functions as the above fibers, are easy to ingest and are user-friendly in preparing processed foods.
In recent years in Japan, processed foods, precooked foods, fast foods and the like have found wider use with the maturity of economical environments and the resulting improvements in food processing techniques and distribution techniques. With this trend, diversified information as to the ingestion of foods has become available, and eating habits to fulfill the nutrient requirements are changing to health-oriented dietary habits contemplated for the prevention of nutrition disorders and degenerative diseases due to eating habits. Especially, people of middle or advanced age and young women have much need for low caloric foods, so that low caloric sweeteners and bulking agents for strong sweetening agents have been developed. Among these, low caloric sweeteners include various indigestible oligosaccharides and sugar alcohols, which nevertheless have many problems with respect to the quality, degree of sweetness, oligosaccharide content and likelihood of causing laxation.
The bulking agent available for use with strong sweetening agents such as aspartame is polydextrose only, whereas polydextrose is ingestible in a limited amount, tastes bitter in an acid condition, is hygroscopic and therefore has problems. In view of the situation described, it has been desired to provide a low caloric bulking agent which fulfills the requirements for use as a food and which is usable for sweeteners and the like with safety.
On the other hand, starch is used in large quantities in various processed foods as a food material. Useful food materials of these types include starch and starch products such as pregelatinized starch, pyrodextrin, derivatives, glucose, corn syrup solids and maltodextrin. However, a majority of these starch products are not higher than 5% in the content of indigestible component and at least 3.9 kcal/g in caloric value, so that among starches and like materials, only pyrodextrin appears useful as a dietary fiber and low caloric material. Heat-treated starch (pyrodextrin) will hereinafter be referred to merely as "dextrin".
We conducted continued research on processes for preparing dextrin, starch hydrolysis processes and processes for preparing indigestible dextrin from dextrin. Based on the results obtained, we have already filed a patent application for an invention entitled "Process for Preparing Dextrin Food Fiber". Our research, subsequently carried out on the physiological activities of the dextrin, revealed that the dextrin had an improvement in intestine conditions, amelioration in hypercholesterolemia, a lowering insulin requirements, a hypotensive effect and a lower caloric value, i.e. effects similar to those of dietary fibers. Based on the finding, we have filed a patent application for the dextrin as a food composition.
We have also investigated the correlation between the structure of dextrin and the content of indigestible fraction thereof and found that the content of indigestible component of dextrin is in inverse proportion to the amount of 1.fwdarw.4 glycosidic linkages of dextrin among other glycosidic linkages thereof. We have further conducted detailed research.
The research thus conducted on various dextrins indicates that the indigestible component is closely related with the quantities of 1.fwdarw.4, 1.fwdarw.6 and like glycosidic linkages, and statistical numerical analyses afforded equations representing a high degree of correlation there-between.
With the prior-art reaction conducted at atmospheric pressure, the velocity of reaction is a function of the time and temperature, i.e., reaction conditions, whereas the heat treatment of starch under increased pressure is entirely different from the conventional reaction in that the correlation between 1.fwdarw.4 and 1.fwdarw.6 linkages and the indigestible fraction can be expressed by a special function, i.e., equation. This is a novel finding we obtained. Nevertheless, the dextrins obtained by conventional techniques are as low as 5 to 30% in the content of indigestible component, contain a colored substance or release a stimulative odor even if obtained by a high-temperature long-time reaction, and are in no way actually usable.